Structural characteristics of as-deposited and crystallized mixed-phase silicon films

Abstract

In this work, we report on the structural characteristics of as-deposited and crystallized mixed-phase silicon films prepared by thermal decomposition of silane in a low pressure chemical vapor deposition reactor. Mixed-phase films consist of crystallites embedded in an amorphous matrix. The size of these crystallites depends upon the surface diffusion length, a parameter quantitatively expressing the potential of adsorbed silicon atoms for surface diffusion. The density of the pre-existing crystallites can be related to the maximum density of critical nuclei, which develops during the deposition of the film. Both variables were quantitatively related to the deposition temperature and rate via physical models reflecting the experimental observations. Values for the parameters associated with these models were extracted by fitting the experimental data to the theoretical equations. Our theoretical analysis is the first to relate quantitatively the structural characteristics of as-deposited mixed-phase films to the prevailing deposition conditions. Mixed-phase films can crystallize in a much shorter time than as-deposited amorphous films, due to the combination of the growth of the pre-existing crystallites and the higher nucleation rate of new crystallites within the amorphous matrix of the mixed-phase film. The crystallization time and final grain size of crystallized mixed-phase films were found to decrease with increasing density of pre-existing crystallites. However, we showed that if composite films are deposited, consisting of a mixed-phase layer and an amorphous layer, the grain size after crystallization could be comparable to that of crystallized as-deposited amorphous films, with the crystallization time of such composite films about threefold shorter. The structure of both as-deposited and crystallized single and composite mixed-phase films was found to be identical for films deposited on both oxidized silicon and Corning Code 1735 glass substrates.